DO NOT BUY THIS! I don't usually write reviews calling an item "a piece of garbage" but this is a PIECE OF GARBAGE!! I bought one, and two of my buddies also bought the same one, they all 3 failed! I used some GH standard solution (PH 7.0) to check that this was calibrated, and it read 7.0 when I tested the solution, and since it only has one set point (7.0) I didn't think to test it in some of the 4.0 PH standard solution. BIG MISTAKE!!! I mixed my nutes and tested them and I kept geting a reading of 6.9-7.1 even after dumping over 40mL of PH down into my 8 liter veg reservoir (drip to waste coco- coir)... I kept getting 6.9-7.1 and I thought for sure something was wrong with my PH Down Solution, because every time I checked the meter in 7.0 solution it read 7.0, like it should. Well I got fed up and I pulled out a brand new bottle of GH standard solution (pink) PH 4.0 and guess what it read... 6.9-7.1!!!!! I tried to use the calibration screw to see if I could get the meter to read 4.0 in the 4.0 solution... the lowest I could get before the screw stopped turning (and then eventually broke because it's a plastic screw head) was 5.9!!! But then I put it back in the 7.0 solution, after all that cranking and it still read 7.0!!! This is the same experience both of my friends had with this same meter!!! I should have taken their advice and steered clear!! Now I am back to buy a more expensive higher quality meter, and all the money I "saved" by purchasing this garbage was wasted!!! (PS when it arrived the cap and the probe had salt residue all over it, like it had been packaged with standard solution to keep the probe moist, but that had long dried out!)Rating: [1 of 5 Stars!]